The anode of a solid oxide fuel cell (SOFC) typically consists of a porous cermet made of nickel and yttria-stabilized zirconia. The nickel component provides electrical conductivity, electrochemical capability and fuel processing capability. The nickel component also enhances the mechanical properties of the cell. Nickel is a relatively unstable metal at high temperature, and in particular it is unstable in the presence of free oxygen at temperatures above approximately 350° C. At normal SOFC operating temperatures of 600° C. to 1000° C., the anode must be subjected to a reducing atmosphere with a partial pressure of oxygen below the nickel-nickel oxide equilibrium level. This allows the nickel to remain in a reduced metallic state.
During SOFC shut down situations, the propensity of nickel to oxidize can cause structural and operational problems. If the nickel anode oxidizes to form nickel oxide, an increase in volume and weight will occur, introducing large stresses in the anode structure. This can result in physical failure of the anode, the electrolyte, or both. Further, after being converted to nickel oxide, the cell is unable to convert chemical energy into electrical energy efficiently and is considered a failed part.
Additionally, if nickel (or nickel oxide) comes into contact with carbon monoxide at temperatures less than about 230° C., nickel carbonyl may form. This substance is highly toxic and potentially fatal if inhaled. Nickel carbonyl is also highly flammable, and is explosive in certain conditions.
Therefore, during a SOFC system shutdown, it is necessary to protect an anode which contains nickel from oxidation and to prevent carbon monoxide containing gas from contacting the anode at temperatures below 230° C. to ensure that no nickel carbonyl is formed. Currently, a number of strategies are employed to protect the anode and to promote a reducing atmosphere around the anode.
In one strategy, an inert gas, such as nitrogen, containing a small amount of a reducing gas, typically hydrogen, can be continually fed into the cell. This strategy is acceptable if a source of the inert and reducing gas is available and the economics and siting justify its use. In most commercial installations, this is an impractical solution because of the quantity of gas required. An array of 8 fuel cell stacks (2-5 kW) may require 45,000 standard liters of reducing gas for a 15 hour shutdown, for example.
Alternatively, the SOFC can be sealed to prevent any oxidizing gas from entering the system. This latter strategy requires hermetic seals and valves, which is technically very difficult to achieve, requiring complex and expensive engineering.
In another strategy, the anode is protected by applying a voltage across the fuel cell, which results in any oxygen in the anode environment being “pumped” across the membrane to the cathode. Although this system may reliably protect the anode, it requires an external power source which may not always be available, especially during an emergency shutdown situation. Further it does not necessarily avert the problem of nickel carbonyl formation and is only available at temperatures where the electrolyte remains active.
Therefore, there is a need in the art for shutdown methods and systems which prevents or minimizes the formation of nickel carbonyl and prevents or minimizes damage to the cell during shutdown or other conditions where anode oxidation may occur.